1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and an image forming method thereof, and particularly relates to some techniques for preventing the back surface and edges of sheets from being stained in the secondary transferring unit.
2. Description of the Related Art
Currently, the most prevalent type of color image forming apparatus employs an intermediate transferring body. Individual color images formed on respective image carriers are sequentially superimposed and transferred onto the intermediate transferring body (belt), and then the superimposed and transferred images are transferred together onto a sheet. In such an image forming apparatus, a secondary transferring member (roller) in contact with the back surface of a sheet is often used to transfer multiple color toner images onto the sheet together.
Also in cases of color printers or color copiers, the density and misalignment of the respective colors with respect to the sub-scanning direction are periodically adjusted in a certain timing. Although there are a variety of specific methods for implementing the adjustment, according to one adjustment method as shown in FIG. 7, upon detection of a predefined action such as switching of the power supply, opening and closing of an outer cover or completion of a jam operation, the adjustment of the density and misalignment of respective colors with respect to the sub-scanning direction is activated.
In this adjustment process, a predefined adjustment pattern (reference pattern) is formed and carried onto an intermediate transferring body. Some sensors disposed along the belt to face the intermediate transferring body are used to detect the density of respective color patterns and/or the misalignment among respective color photoconductors. The developing bias and/or others are adjusted to some proper values and/or the color misalignment is corrected by tuning (adjusting) the printing timing onto the photoconductors.
In arrangements without any contact and release mechanism for making and releasing contact between the secondary transferring member and the intermediate transferring body, toner composing a density adjustment pattern and/or a position adjustment pattern becomes attached to the secondary transferring member, and consequently the back surface and/or the image surface of a sheet may be stained in secondary transferring operations at the image formation and/or both sided recording operations, respectively.
In order to address the problem, Japan Laid-Open Patent Application No. 2004-272206 discloses an image forming apparatus for collecting toner from the secondary transferring member to the intermediate transferring body efficiently by applying positive and negative bias voltages to the secondary transferring roller alternately.
According to the image forming apparatus as disclosed in this document, the toner is moved toward the intermediate transferring belt through operations of an electric field applied to the transferring roller. However, it is impossible to deliver the toner to the intermediate transferring body completely even through intensive operations of the electric field. Since only electrically-charged toner is electrostatically movable, weakly charged toner and non-charged toner cannot be electrostatically moved. When the weakly charged toner and/or the non-charged toner is attached to the secondary transferring member, it is hard to move the toner to the intermediate transferring body even through electrostatic application of an electric field, and thus the back surface of a sheet may be stained during passage. In addition, Japan Laid-Open Patent Application No. 2004-272206 does not take environmental variations into consideration, and thus if the environment varies, the optimum cleaning bias is not applied to the secondary transferring member, resulting in reduced efficiency of the toner collection.